System and method for performing tear film structure measurement

ABSTRACT

Apparatus and methods are described for performing structure measurement on a tear film of an eye of a subject. At least a portion of a surface of the tear film is illuminated using a broadband light source. A spectrum of light of the broadband light that is reflected from at least one point of the tear film is measured, using a spectrometer. Color information for a plurality of points of the tear film is obtained, by imaging a field of view of the tear film using a color camera. Using a processing unit, data from the color camera and data from the spectrometer that are indicative of characteristics of the tear film are received, and based upon a combination of the data received from the color camera and the data received from the spectrometer, an output is generated that is indicative of a structure of the tear film.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 15/227,839 toArieli (published as US 2016/0338585), filed Aug. 3, 2016, now U.S. Pat.No. 9,757,027, which is a continuation-in-part of InternationalApplication No. PCT/IL2015/050232 to Arieli (published as WO 15/132788),filed Mar. 4, 2015, which claims priority from U.S. ProvisionalApplication No. 61/948,579 to Cohen, filed Mar. 6, 2014. Each of theaforementioned applications is incorporated herein by reference.

FIELD OF EMBODIMENTS OF THE INVENTION

The present invention relates to optical layers measurements, and moreparticularly, to a method and apparatus for measuring the layers of thetear film.

BACKGROUND

In recent years, the diagnosis of dry eye has become an importantsubject of ophthalmologic diagnosis.

The diagnosis is conventionally performed by vital staining test, butthis test requires the use of chemical eye drops and is painful to theexaminee.

There are several methods for diagnosis of dry eye without contact.

U.S. Pat. No. 6,299,305 to Miwa describes an ophthalmic apparatus formeasuring the dryness of a cornea of an eye to be examined by projectinglight onto the cornea and detecting the reflected light from the tearfilm. The device measures the time-varying signal to determine thechanges in the dryness of tear film. The signal is the fluorescencelight reflected by a fluorescein being spread over the tear film.

At J. Opt. Soc. Am. A, 15, 268-275; IOVS, October 2000, 41, 11,3348-3359 and IOVS, January 2003, 44, 1, 68-77 describewavelength-dependent optical interferometers that have been developedfor in-vivo aqueous tear film and contact lens thickness analysis. Theinstruments described in the publications are of similar design and arecapable of measuring the thickness of the pre-corneal or pre-lens tearfilm aqueous+lipid layer thickness, post-lens tear film aqueousthickness among contact lens, contact lens thickness and cornealepithelial thickness. The instruments can also measure the thinning orthickening rates of the various tear film layers during normal blinkingand between blinks or over time.

According to the described approach, the light reflections from theocular surface arise from the combined aqueous+lipid layer and the lipidlayer alone since the wavelength-dependent fringes from the aqueouslayer only cannot be observed. Thus, the tear lipid layer thicknessneeds to be measured separately and subtracted from the combinedaqueous+lipid thickness to derive aqueous-only layer thickness. Thisapproach does not take into account the other layers above or below theaqueous layer such as the lipid layer and the microvilli Mucin layer. Italso does not take into account the possible interaction between all thelayers in the stack, their combined interference and their relativeintensities. In order to have a proper and an accurate measurement onemust use a suitable physical model that takes all interfaces reflectionsand interference into account. The wavelength-dependent oscillations arecreated by the interactions of all combinations of reflections from thecombined Microvilli height+aqueous+lipid layers, so that manyinterference frequencies can co-exist simultaneously. In the case ofvery thin layers whose thickness is the order of the wavelength of thelight, such interference cannot be simply deduced from the power densitydue to resolution limits and there is a need to measure the absolutereflection changes across the spectrum and apply proper physicalmodeling. The model also must take into account the scattering of lightfrom the lower interface below the aqueous layer and the gradient changeof the Mucins concentration that creates gradual optical propertieschanges. In addition, the optical setup described, does not work inoptimal conditions to measure the tear layers. Other important aspectsof the system such as measuring the evaporation rate and auto-focusingwith fast feedback to allow reasonable measurement success rate aftersmall movements or blinking, are missing.

In J. Opt. Soc. Am. A, 23, 9, 2097-2103, 2006, a method for recordinginterference images from the full thickness of the precorneal tear film(PCTF) is described. Simultaneous images are recorded by two videocameras. One camera responds to broadband spectral illumination andrecords interference from the superficial lipid layer of the tear filmwhile the other camera uses narrowband illumination and recordsinterference from both the lipid layer and the full thickness of thePCTF. Thus the full-thickness interference fringes are derived from thedifference between the narrowband and the broadband images. In thismethodology the low amplitude reflection from the aqueous layer isderived from the narrowband light. However there is no indication aboutthe absolute values of the thickness since the fringe contrast is anindication only for quarter wavelength change in the full thickness.

US 2008/0273171 to Huth describes a method of diagnosing dry eye bytaking multiple measurements from a single point of the subject's eye.Each measurement uses an interferometer and calculates the thicknessesof the aqueous and the lipid layers by comparing the reflection of lightfrom the eye to reflection calculated by an empirical equation. The sameequation can also be used to measure the thickness of the lipid layer,thus, aqueous-only layer thickness is calculated by subtracting themeasured lipid-only layer thickness from the combined aqueous+lipidlayer thickness. However, in this procedure there is no impact to theexistence of under layers or lower interface properties such as theepithelial or microvilli structures thus the confidence levels for theaccurate values obtained are questionable.

US 2009/0225276 to Suzuki describes a method to measure the amount oftear fluid. A low magnification light source illuminates the outermostlayer of a tear film on the cornea of a subject's eye. The interferencestripes pattern created by the lipid film on the cornea is displayed ona monitor. To measure the amount of tear fluid, different light-emittingelements of a high-magnification light source are used to irradiate thetear fluid meniscus that has accumulated on the lower eyelid portion ofthe anterior ocular segment. The light reflected from the surface of themeniscus forms separated images of the light-emitting elements on amonitor with some interval between the images. The amount of tear fluidcan be quantitatively measured by measuring the interval between theimages. This method relies on the connection between the meniscus andthe center of the cornea and therefore suffers from inaccuracy.

In US 2012/0300174 to Yokoi the tear film lipid layer on a cornea of aneye is described as being illuminated by a white light source and isimaged by a color camera. The image of the tear film lipid layer isprocessed, and the initial spread speed of the tear film lipid layer andthe time until the tear film lipid layer is broken are measured.

In U.S. Pat. No. 7,281,801 to Wang the thickness and the dynamics of atear film layer and the heights of tear menisci around upper and lowereyelids of an eye are described as being measured by acquiring aplurality of images between consecutive blinks of the eye using opticalcoherence tomography (OCT). The plurality of reflectivity profiles fromthe OCT images are aligned and averaged and the difference between afirst peak and a second peak of the average reflectivity profile ismeasured to determine the thickness of the tear film layer. This methodrelies on the connection between the meniscus and the center of thecornea and therefore suffers from inaccuracy.

In U.S. Pat. No. 8,192,026 to Gravely the relative thickness of thelipid layer component of the precorneal tear film on the surface of aneye is described as being measured by illuminating the eye using aLambertian broad spectrum light source covering the visible region. Thelight is specularly reflected from the lipid layer and undergoesconstructive and destructive interference in the lipid layer. Thespecularly reflected light is collected and the interference patterns onthe tear film lipid layer are imaged on to a high resolution videomonitor. The lipid layer thickness is classified on the basis of themost dominant color present in the interference pattern. This methodsuffers from basic problems of “2-π ambiguity” or “order skip” and thusprevents uniqueness of the measurement.

Thus, there is required a simple and reliable method and system that canaccurately measure both the lipid and the aqueous layers, twodimensionally to enable the determination of the evaporation ratesrequired for diagnosing dry eye phenomena.

SUMMARY OF EMBODIMENTS

It is therefore an object of the present invention to provide a simpleand reliable optical system and method that can measure the lipid andthe aqueous layers in large area.

This object is realized according to the invention by a system andmethod having the features of the respective independent claims.

According to the one embodiment of the present invention a combinationof a spectrometer and an interferometer is used for measuring the lipidand the aqueous layers in large area where the measurement accuracy andspeed are also suitable for the measurement of the evaporation rate.

According to another embodiment of the present invention a combinationof a spectrometer and a color camera is used for measuring the lipid andthe aqueous layers in large area.

According to another embodiment of the present invention a spectrometeris used to measure the Microvili depth and Mucin gradient profile.

According to another embodiment of the present invention the combinationof a spectrometer and color camera is used for measuring the lipidcontinuity and break up time.

There is further provided, in accordance with some applications of thepresent invention, a system for performing tear film structuremeasurement, the system including:

a broadband light source configured to illuminate the tear film;

a spectrometer for measuring respective spectra of reflected light fromat least one point of the tear film;

a color camera configured for large field of view imaging of the tearfilm so as to obtain color information for all points of the tear filmimaged by the color camera; and

a processing unit coupled to the camera and to the spectrometer andconfigured for (i) calibrating the camera at the at least one pointmeasured by the spectrometer so that the color obtained by the camera atsaid at least one point matches the color of the spectrometer at thesame point, and (ii) determining from the color of respective points ofthe calibrated camera thicknesses of one or more layers of the tear filmat the respective points.

In some applications, the system further includes a flat objectiveFresnel lens that is configured to converge a large range of angles to apoint and is disposed a short distance from the tear film.

In some applications, the system further includes an autofocusingmechanism for focusing the color camera and the spectrometer.

In some applications, the system further includes a mechanism forcentering a cornea of the subject onto optical axes of the spectrometerand the color camera, the mechanism including a projector that isconfigured to project a given pattern onto the subject's cornea.

In some applications, the spectrometer is configured to measure therespective spectra of reflected light from the at least one point of thetear film using a measurement time of between 20 and 300 milliseconds.

In some applications, the system further includes an aperture that isconfigured to facilitate detection of tear film sub-micron level surfacetopography by causing narrow angles of the reflected light to bereceived by the color camera.

In some applications, the processing unit is further configured tocompute respective rates of change of thicknesses of the one or morelayers of the tear film at different points along the tear film obtainedover a known time interval.

In some applications, the system further includes one or more opticalelements selected from the group consisting of: lenses, Fresnel lenses,annular aperture filters, polarizers, spatial light modulators (SLMs),and diffractive optical elements (DOEs).

In some applications, the system further includes an interferometerhaving a pair of mirrors disposed in a path selected from the groupconsisting of: an illumination path of the system and an imaging path ofthe system, and configured to modulate the spectrum of the light sourceby changing the optical path difference between the mirrors.

In some applications, the interferometer includes a spatial lightmodulator (SLM) disposed in an arm thereof.

In some applications, the processing unit is coupled to theinterferometer and configured for (i) calibrating consecutively theinterferometer by computing the optical path difference between themirrors of the interferometer at at least one of the points measured bythe spectrometer, (ii) using the calibrated interferometer to modulatethe spectrum of the light source by changing the optical path differencebetween its mirrors, (iii) obtaining a calibrated interferogram atrespective points of the tear film, and (iv) using the calibratedinterferogram to determine thickness of one or more layers of the tearfilm at the respective points.

In some applications, the spectrometer further includes a deflectingelement for scanning different tear film locations.

In some applications, the system further includes a narrow band filterbetween the light source and the camera for producing interferencefringe patterns characteristic of different layers of the tear film.

In some application, the spectrometer is used to correlate a fringepattern in a known location or pixel with a respective thickness of eachlayer of the tear film and thereby derive the respective thickness ofeach layer of the tear film without scanning the tear film.

In some applications, the spectrometer is configured to measure therespective spectra of reflected light from the at least one point of thetear film using a measurement spot size of between 40 microns and 300microns.

In some applications, the spectrometer is configured to measure therespective spectra of reflected light from the at least one point of thetear film using a measurement spot size of between 100 microns and 240microns.

There is further provided, in accordance with some applications of thepresent invention, a method for performing tear film structuremeasurement, the method including:

illuminating the tear film with a broadband light source;

using a spectrometer to measure respective spectra of reflected lightfrom at least one point of the tear film;

imaging on a color camera a large field of view image of the tear filmso as to obtain color information for all points of the tear film;

calibrating the camera at the at least one point measured by thespectrometer so that the color obtained by the camera at said at leastone point matches the color of the spectrometer at the same point; and

determining from the color of respective points of the calibrated camerathicknesses of one or more layers of the tear film at the respectivepoints.

In some applications, the method further includes computing respectiverates of change of the thicknesses of the one or more layers of the tearfilm at different points along the tear film obtained over a known timeinterval.

In some applications, the method further includes:

(i) using a narrow band filter to obtain an interference pattern due tothickness non uniformity of the one or more layers of the tear film;

(ii) determining absolute values of the thicknesses of the one or morelayers of the tear film at the overlapping locations of the interferencepattern and spectrometer measurement points;

(iii) applying the absolute values at overlapping locations to determinethe absolute values of the thicknesses at neighboring pixels that alsohave interference patterns.

In some applications, the method further includes using electromagneticsimulation in order to fit measurement results to a biological model ofstack of tissues and liquid layers.

In some applications, the method further includes autofocusing the colorcamera and the spectrometer.

In some applications, using a spectrometer to measure respective spectraof reflected light from at least one point of the tear film includesusing a measurement time of between 20 and 300 milliseconds formeasurements performed by the spectrometer.

In some applications, the method further includes:

Fourier transforming the spectra of the reflected light so as to obtain,for each of point measured by the spectrometer, a respectiveinterferogram; and

using the interferograms to determine the thickness of the one or morelayers of the tear film at the corresponding point.

In some applications, the method further includes computing a respectiverate of change of the thicknesses of the one or more layers of the tearfilm at different points along the tear film obtained over a known timeinterval.

In some applications, the method further includes modulating a spectrumof the light source by changing an optical path difference between apair of mirrors of an interferometer.

In some applications, the method further includes:

(i) calibrating the interferometer by computing the optical pathdifference between the mirrors of the interferometer at at least one ofthe points measured by the spectrometer,

(ii) using the calibrated interferometer to modulate the spectrum of thelight source by changing the optical path difference between the mirrorsof the calibrated interferometer,

(iii) using the measured reflected light to obtain a calibratedinterferogram at respective points of the tear film, and

(iv) using the calibrated interferogram to determine a thickness of theone or more layers of the tear film at the respective points.

In some applications, using a spectrometer to measure respective spectraof reflected light from at least one point of the tear film includesusing a measurement spot size of between 40 microns and 300 microns.

In some applications, using a spectrometer to measure respective spectraof reflected light from at least one point of the tear film includesusing a measurement spot size of between 100 microns and 240 microns.

There is further provided, in accordance with some applications of thepresent invention, a method including:

acquiring a plurality of measurements of a thickness of a layer of atear film of an eye of a subject, using a measuring device selected fromthe group consisting of: a spectrometer, and an interferometer;

identifying a time period over which there is a change in the thicknessof the layer of the tear film that has a negative correlation with time,the correlation having a correlation value that passes a threshold; and

identifying a rate of thickness change of the layer by determining arate of thickness change of the layer over the identified time period.

There is further provided, in accordance with some applications of thepresent invention, a method including:

acquiring a plurality of measurements of a thickness of a layer of atear film of an eye of a subject over a given time period, using ameasuring device selected from the group consisting of: a spectrometer,and an interferometer;

determining a stability level of the layer by determining a ratiobetween an indication of variance of the thickness over the given timeperiod, and an average of the thickness over the given time period; and

at least partially in response thereto, determining a level of thehealth of the tear film.

There is further provided, in accordance with some applications of thepresent invention, a method including:

acquiring a plurality of measurements of a thickness of a layer of atear film of an eye of a subject, using a measuring device selected fromthe group consisting of: a spectrometer, and an interferometer;

subsequent to the subject having blinked, identifying that a breakup ofthe layer of the tear film has occurred, in response to determining thethickness of the layer has fallen below a threshold;

determining a breakup time, by measuring the time between the subjecthaving blinked and the occurrence of the breakup event; and

at least partially in response thereto, determining a level of health ofthe tear film.

In some applications, identifying that the breakup event has occurredfurther includes:

in response to identifying that a potential breakup event has occurredin a given area;

comparing a variance in the thickness of the layer in the given area tovariances of thickness of the layer in areas that are in a vicinity ofthe area; and

confirming that the breakup event has occurred at least partially inresponse thereto.

There is further provided, in accordance with some applications of thepresent invention, a method, for use with an optical system thatincludes a camera, the method including:

detecting surface topography of a portion of a curved surface of anobject by:

-   -   directing a beam toward the surface from an angle of incidence        of more than 50 degrees to an optical axis of the camera;    -   receiving light reflected from the surface with the camera, via        a narrow-angle aperture; and    -   using a computer processor:        -   detecting one or more darkened regions in the received            light; and        -   detecting the surface topography in response to the detected            darkened regions.

In some applications, receiving light reflected from the surface withthe camera, via the narrow-angle aperture includes receiving lightreflected from the surface with the camera, via a narrow-angle aperturethat defines an angle of less than 3 degrees.

In some applications, the method further includes detecting surfacetopography of a region of the surface that is in a vicinity of theoptical axis of the camera, by:

detecting a spectrum of broadband light that is reflected from theregion, using a spectrometer; and

analyzing the detected spectrum, using the computer processor.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is a schematic illustration of an optical system, in accordancewith some applications of the present invention;

FIG. 2 is a schematic illustration of a subject's eye, the lipid layerof the tear film of the subject's eye including a collapsed region uponwhich measurements are performed, in accordance with some applicationsof the present invention;

FIG. 3A is a schematic illustration of an optical element of an opticalsystem, in accordance with some applications of the present invention;

FIG. 3B is a schematic illustration of light being reflected from thesurface of an object, in accordance with some applications of thepresent invention;

FIG. 4 is a graph showing a typical variation of the thickness of anaqueous layer as a function of time, as measured in accordance with someapplications of the present invention; and

FIG. 5 is a graph showing the thickness of the lipid layer of the tearfilm as function of time, as measured in accordance with someapplications of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a preferred embodiment according to the present inventionin which an optical system 10 is used for measuring a biological tissueor a biological substance such as the lipid and the aqueous layers of aneye 210 over a large area. The system comprises a combination of aspectrometer 250 and/or an interferometer 140 and a color camera 270. Itis well known that the contrast of the fringes (“spectral oscillations”)originated from the aqueous layer is always considerably less than thecontrast that can be obtained from the lipid layer. This fact is due tothe anti-reflection coating effect caused by the mucus layer under theaqueous on the cornea and the Microvili scattering. However, this effectis much stronger in the visible range of the spectrum (400-800 nm) thanin the near IR.

This effect can be overcome by combining (a) a spectrometer and/or aninterferometer in the near infrared and visible region (NIR-VIS) and (b)a color camera. The spectrometer and/or interferometer provides theinformation of the fringes in the NIR and the VIS and the cameraprovides the information of the fringes in the VIS. (It is noted thatthe interferometer is typically used in conjunction with an imagingcamera, such as camera 270.) From the information of the fringes in theNIR obtained by the interferometer and/or the spectrometer, thethickness of the aqueous layer is calculated. From the information ofthe fringes in the VIS by the camera, the thickness of the lipid layeris calculated. The information of the fringes in the VIS obtained by theinterferometer and/or the spectrometer is also useful for calibratingthe color camera. The accurate color of the point(s) where theinterferometer and/or the spectrometer measures, can be calculated andthe result can be used for calibrating the camera.

In the present embodiment which is described only for illustration,there are two paths; the illumination path and the imaging path. In theillumination path, the light emanates from a broadband light source 100and is collimated by lenses 110 and 120. The light is folded by a mirror130 or optionally by an interferometer 140 with movable mirrors. Theinterferometer can alternatively be disposed in the imaging path (to bedescribed below) instead of the illumination path. (It is noted thatFIG. 1 shows interferometer 140 disposed in both the illumination pathand in the imaging path. However, typically, the interferometer isdisposed in either the illumination path, or the imaging path.) Thelight is focused by the lens 150 on at least two grids 160 andcollimated by the lens 155. The grids 160 are imaged on the cornea. Thegrids 160 are used for autofocusing and positioning the cornea in acertain determined distance from the system by examining the sharpnessof the images of the grids on the cornea. The light passes throughbeam-splitter 170.

In another mode the autofocusing can be done directly on the image offeatures that exists on the cornea (e.g. lipids topography or simply theIris). The light is directed to the cornea by a focusing optical element(e.g., a lens) 200. The focusing optical element 200 may be any kind offocusing optical element such as a compound lens, a Fresnel lens,Diffractive Optical element, etc. The light is focused to theapproximated focal point of the concave mirror formed by the cornealsurface such that it is reflected back at a small angle relative to theoptical axis. The reflected light is gathered by the central part of thefocusing optical element 200 or by an additional optical element placedin the central part of the focusing optical element 200. Optionally,other optical elements 180 may be disposed in the light path, such as apolarizer, which can contribute for proper background removal. For someapplications, optical element 180 is a narrow aperture optical element,as described in further detail hereinbelow, with reference to FIG. 3A. Areticle 190 may also be disposed within the light path to serve as atarget for directing the subject's gaze. In the imaging path, the lightreflected from the cornea is partially reflected by the beam splitter220 and focused on the camera 270, by lens 260, to image the cornea. Thetransmitted light is focused on the spectrometer 250, using lens 230,and analyzed.

In order to increase the accuracy of the autofocusing and/or to centerthe measured cornea relative to the optical axis of the camera and/orthe spectrometer, a known pattern such as a circle or a square or someother structured light pattern may be projected onto the cornea using aprojector. Due to its curvature, when the cornea is decentered relativeto the optical axis, the image of the projected pattern is distorted.For some applications, this distortion is processed and used to centerthe cornea in real time.

As mentioned above, the information of the fringes in the NIR providedby the spectrometer and/or interferometer is used for calculating thethickness of the aqueous layer, and the information of the fringes inthe VIS provided by the camera is used for calculating the thickness ofthe lipid layer. The information of the fringes provided by theinterferometer and/or the spectrometer in the VIS is used forcalibrating the color camera.

In an alternative mode the combined VIS-NIR spectrum can be analyzedusing 3D electromagnetic simulation known in the art such as FTDT(Finite-Difference Time-Domain) or RCWT (Rigorous Coupled Wave Theory)or Green Function based calculations. The measured reflection of lightfrom the cornea can be compared iteratively to a simulated measurementuntil a final fit is achieved for best corneal structure parameters. Insuch way both the thicknesses of the lipid layer, aqueous layer, mucinlayer and microvilli roughness can be calculated simultaneously.

When the interferometer 140 is disposed in the imaging path before thecamera 270 and the optical path difference (OPD) between theinterferometer's mirrors is increased constantly, an interferogram isobtained for each point of the camera image. The Fourier transform ofthe interferogram provides the respective spectrum at each point of theimage. The spectra can be analyzed for calculating the thickness of thelipid and aqueous layers. Since the spectrum in at least one point ofthe image is obtained also by the spectrometer, the spectra obtained bythe spectrometer and the spectra obtained by the camera can be comparedand the movements of the interferometer's mirrors can be calibrated.

As mentioned above, the interferometer 140 can also be disposed withinthe illumination path. In this embodiment, the light from the lightsource is modulated by a cosine function as a function of the OPDbetween the interferometer's mirrors. Fourier transforming the intensityof light at each point of the image as a function of the OPD alsoobtains the light spectra. The light spectra can be analyzed forcalculating the thickness of the lipid and aqueous layers. Since thespectra of the image are also obtained by the spectrometer, the spectraobtained by the spectrometer and the spectra obtained by theinterferometer can be compared and the movements of the interferometer'smirrors can be calibrated.

The provision of the interferometer 140, either in the illumination pathor in the imaging path, and the combination of the interferometer, thespectrometer and the color camera, add some advantages:

When the OPD between the interferometer's mirrors is increasedconstantly, an interferogram is obtained for each point of the image atthe camera. At each point of the image, the interferogram is recordedseparately by the different color detectors of the color camera. Sincethe Fourier transform of the interferogram is the spectrum of thereflected light reaching the detector, all color detectors at each pointof the camera can be calibrated absolutely using the spectral dataobtained. The main advantage is that this calibration includes theoverall spectral response of the whole system, the light source, theintermediate and the detector for each pixel of R/G/B differently.

The calibration of the response of the color detectors either by thespectrometer, the interferometer or both, is very critical for the lipidmeasurements where the sensitivity of the color change as a function ofthe lipid layer thickness is very high especially in the shorterwavelengths.

Another advantage of adding the interferometer and/or the spectrometerto the system is that the color detectors in the color camera integrateall reflected light, i.e. the specular and the diffuse light which mayoriginate from much deeper interfaces. Thus, the thicknessescalculations based on the color camera alone may be inaccurate. However,in the spectrometer and the interferometer since the diffuse lightcontributes only to the DC level of the signal and not to the modulatedsignal, and thus can be omitted, this can increase the accuracy of thecolor and the thicknesses calculations based on it.

The combination of spectrometry and interferometry can also be used forthe purpose of achieving a larger range of thickness measurements. Ingeneral, the spectrometer has a very good signal to noise ratio (SNR)but is limited to measuring thin films only due to its limitation on thespectral resolution. The high SNR enables measuring thin turbid layersvery accurately (sub-nanometers). On the other hand the interferometercan measure several orders of magnitude higher and therefore its rangecan be complementary to the spectrometer range. In those cases where theinterferometer detects thin turbid layer at a certain depth, theautofocus mechanism can shift the best focus position to this certaindepth and the spectrometer can measure the reflectance from the thinturbid layer with higher SNR.

In still another embodiment, a spatial light modulator (SLM) can beintegrated in one of the interferometer's arms. By applying severalphase delays to the light by the SLM, the thicknesses of the thin layerscan be calculated using Phase Shift Interferometry algorithms. The SLMmay be based on moving micro-mirrors, LCD or any other method known inthe art.

In order to increase the number of the points of the spectrometer'smeasurements, a deflecting element 240 may be disposed in the light pathto the spectrometer. The deflecting element 240 deflects the incominglight from the cornea in such a way that at each time the light fromdifferent points of the cornea is analyzed by the spectrometer. In thisembodiment, the calculations of the aqueous layer thickness areperformed at several points and the calibration of the color camera isperformed at several points of the image.

As mentioned above some other optical elements 180 may be added to thelight path, such as polarizer, or a narrow aperture optical element,etc. The addition of a polarizer and/or a narrow aperture opticalelement may improve the signal in the following terms:

The non-specular reflected light is blocked.

The light reflected by the layers under the cornea may be blocked sincethese layers depolarize and rotate the polarization of illuminatinglight and/or are reflected in non-specular manner. In particular, thepolarizer may block the light reflected from the iris of the eye.

Another working mode of the system uses a narrow band filter between thelight source and the camera. In this case the image will haveinterference patterns in the form of fringes that are obtained byinterfering light beams reflected from the different interfaces of thelayers of the tear film. These layers may have thicknessnon-uniformities. The combination of non-uniformities of the layersthicknesses obtained from the interference patterns with the informationobtained at specific discrete points from the accurate spectrometrymeasurements can give the following advantages:

1) a continuous full image with absolute thickness values per pixel;

2) the option to overcome the 2π ambiguity of the interference cycles.

For some applications of the present invention, thickness and/orspectral measurements of a thin biological layer are performed using oneor more measurement parameters described hereinbelow. Typicallymeasurements are performed on a thin biological layer in order todetermine the thickness of the layer, and/or the changes over time of aparameter (such as, the thickness) of the layer. For some applications,thickness and/or spectral measurements of a thin biological layer, suchas a tear film layer, are acquired using a spectrometer and/or aninterferometer, using a measurement spot size of more than 40 micronsand/or less than 240 microns (e.g., 40-240 microns), as described infurther detail hereinbelow. For some applications, the measurements areperformed over a time period of more than 20 ms and/or less than 300 ms(e.g., 20-300 ms), also as described in further detail hereinbelow.

It is noted that the term “spot size” when used in conjunction withspectrometric measurements should be interpreted as meaning the diameterof the area of the detected object from which reflected light isreceived by the spectrometer in a given spectrometric image. When usedin conjunction with an interferometric measurement, the term “spot size”should be interpreted as meaning the diameter of the area of thedetected object corresponding to pixels that are binned together witheach other in the interferometric measurement. As noted hereinabove,typically the interferometer is used together with an imaging camera,such as camera 270. The term “sampling size” may be used interchangeablywith the term “spot size”.

The thin biological layer typically includes one or more sub-layers. Forexample, the tear film typically includes tear film inner layers, suchas the lipid and/or the aqueous layers, and/or one or more delicatemembranes, such as the basement membrane and/or the inner limitingmembrane. A thin biological layer as described herein may include thetear film and/or any one of the aforementioned constituent layers of thetear film. For some applications, such measurements are performed usingthe optical system described hereinabove with reference to FIG. 1. Forsome applications, a combination of two or more measurements isperformed on the tear film, e.g., using techniques describedhereinabove. For example, a high quality and high resolution spectraland/or interferometric measurement of a single spot (or a plurality ofspots) may be performed, together with high quality and largefield-of-view imaging of the reflection from a given inner layer of thetear film. For some applications, measurements as described herein areperformed using a spectrometer, an optical camera, an interferometer,and/or a different imaging device, without using other componentsbelonging to the optical system shown in FIG. 1. Measurements aretypically performed in order to determine clinical parameters that areindicative of the root cause of a dry eye diagnosis and/or the health ofthe tear film. For example, such parameters may include blink rate, tearbreak up time, variation of lipid thickness with time, lipid uniformity,aqueous layer thickness, evaporation rate, etc. For some applications,two or more of the following parameters are measured simultaneously:aqueous flow rate, aqueous layer thickness, lipid layer integrity, andevaporation rate.

Typically, interferometric and/or spectrometric measurements (such asaqueous flow rate measurements) are performed at a frequency of morethan 2 (e.g., more than 5) and/or less than 50 measurements per second(e.g., 2-50, or 5-50 measurements per second). Further typically, suchmeasurements are performed using a maximal value of irradiance of morethan 4 and/or less than 20 mw/cm^2, e.g., between 4 and 20 mw/cm^2. Thisis because an irradiance of approximately 20 mw/cm^2 is the maximumpermitted eye exposure in the visible wavelength range. At this level ofirradiance, in order to provide a signal-to-noise ratio that issufficient to provide an interference pattern that can be resolved, itis typically desirable that the interferometer and/or spectrometer spotsize is at least 40 microns.

Further typically, for spectroscopic and/or interferometricmeasurements, it is desirable that the peak of the interferencereflection as a function of the wavelength is similar over the entiremeasurement spot. Therefore, it is typically desirable that the changeof the layer thickness within the measurement spot is less thanapproximately one-tenth of the mean wavelength. For example, for abroadband illumination with large spectrum of wavelengths which has amean wavelength of 800 nm, it is desirable that within the measurementspot, the variation in the thickness of the layer that is being measuredis less than 80 nm. It is typically a reasonable assumption that thethickness of the aqueous layer of the tear film has a uniformity of 330nm per millimeter. Therefore, for some applications, when performingmeasurements on such a layer, a spot size of less than 240 microns isused (since 80 nm (the maximum desired variation in thickness) dividedby 330 nm per millimeter (the variation per millimeter of the layerthickness) is 0.24 millimeters, i.e., 240 microns).

In view of the constraints described in the above paragraphs, typicallywhen performing a thickness and/or a flow measurement on the aqueouslayer based on an interference signal (e.g., using an interferometerand/or using a spectrometer), a spot size of more than 40 microns,and/or less than 240 microns (e.g., 40-240 microns) is used.

Reference is now made to FIG. 2, which is a schematic illustration of asubject's eye 300, the lipid layer of the tear film of the subject's eyeincluding a collapsed region 302, upon which measurements are performed,in accordance with some applications of the present invention. Breakupof a layer's integrity, for example, the collapse of the continuity ofthe thin lipid layer of the subject's tear film may occur periodically,and natural mechanisms, such as the blinking, may repair the film. Inaccordance with some applications of the present invention, parametersare measured that relate to the collapsed lipid layer of the subject'stear film. Many mechanisms can lead to a breakup, especially in acomplex multi layers fluid such as the tear film.

Normally the collapse of the continuity of the thin lipid layer involvesa local collapse of an underlying layer (such as the aqueous layer), orthe absence of a mucin interface layer at a specific location. In suchcases, portions of the lipid layer that overlie the collapsed underlyinglayer may form a net structure, as shown in FIG. 2. The net structureincludes curves 304 of the net which have collapsed into the space leftby the collapsed underlying layer(s), and areas 306 between the curvesof the net which have not collapsed. (It is noted that, for someapplications, collapsed portions of the lipid layer form shapes otherthan that which is schematically illustrated in FIG. 2) In order toperform measurements on region 302, an imaging system with highresolution and with a small measurement spot size is typically used.Typically the spot size is constrained by the following constraints:

(a) It is desirable that the spot size be sufficiently small such thatthe illumination integration within the spot area will be sensitive tothe development of new collapsed curves, and/or changes to the area ofcollapsed curves 304 relative to the total area that is within the spot.Typically, the curves of the net have a width of between 10 and 30microns, and the spectrometer and/or interferometer is sensitive tochanges that encompass 10 percent of the area of the spot. Therefore,typically the spot size is less than 300 microns. Typically, the spotsize is less than 240 microns, due to the non-uniformity of the aqueouslayer, as described hereinabove.

(b) It is desirable that the spot size be sufficiently large so as tocapture curves of the net, if such curves are present. Therefore, it istypically desirable that the spot size is greater than 100 microns.

In accordance with the description hereinabove, typically spectrometricmeasurements are performed using a measurement spot size that isconstrained by one or more of the following considerations:

-   -   In order to perform thickness measurements on the aqueous layer        of the tear film, using a spectrum of wavelengths having a mean        wavelength of approximately 800 nm, the spot size is typically        less than 240 microns. Typically, this is such that the change        of the layer thickness within the measurement spot size is less        than approximately one-tenth of the mean wavelength.    -   In order to perform aqueous flow rate measurement, a minimal        spot size of 40 microns is typically used, in order to provide a        signal-to-noise ratio that is sufficient to provide an        interference pattern that can be resolved.    -   In order to perform integrity measurements on the lipid layer of        the tear film, a minimal spot size of 100 microns is used, in        order to be sufficiently large so as to capture curves of the        net that is formed by the lipid layer when the lipid layer is        collapsed. Furthermore, in order to be sufficiently small such        that the illumination integration within the spot area will be        sensitive to the development of new collapsed curves, and/or        changes to the area of collapsed curves 304 relative to the        total area that is within the spot, the spot size is typically        less than 300 microns, e.g., less than 240 microns.

Accordingly, in accordance with some applications of the presentinvention thickness and/or spectral measurements of a tear film areacquired using a measurement spot size of more than 40 microns (e.g.,more than 100 microns), and/or less than 300 microns (e.g., less than240 microns), for example, 40-300 microns, or 100-240 microns, such aspot size being suitable for most of the measurement requirementsdescribed above. Typically, the measurements are acquired using aspectrometer and/or an interferometer. For some applications, opticalsystem 10 as shown in FIG. 1 is used.

Reference is now made to FIGS. 3A and 3B, which are schematicillustrations of eye 210 being illuminated, in accordance with someapplications of the present invention. As described hereinabove, forsome applications, in addition to performing spectroscopic and/orinterferometric measurements on the tear film, the tear film is imagedusing camera 270 of optical system 10 (shown in FIG. 1). For someapplications, the optical system has a large field of view but with fineresolution, for example, in order to capture local collapse phenomena ofone or more of the underlying layers.

Typically, a curved surface of an object, e.g., the surface of thecornea of eye 210, is illuminated from a broad angle of incidence (e.g.,an angle of incidence of more than 50 degrees relative to the opticalaxis of camera 270). For some applications, the surface is illuminatedusing a point source. For some applications, the optical system isconfigured to perform ray tracing such that each angle out of theoverall angle that is directed to the curved surface (e.g. the surfaceof the cornea of the eye) is directed toward a respective portion of thecurved surface. Light is thereby focused onto the surface (e.g., usinglens 200), such that a respective narrow angle illumination beam isfocused upon respective portions of the surface, as shown schematicallyin FIG. 3A. For some applications, before passing through lens 200, theincident light passes through a collimator, such that light beams thatare incident upon lens 200 are parallel to each other, as shownschematically in FIG. 3A.

Light is collected from the object using a narrow-aperture opticalelement 180. Typically, the narrow-aperture optical element collectslight beams that define an angle of more than 1 degree (e.g., more than2 degrees) and/or less than 3 degrees, e.g., an angle of between 1 and 3degrees, or an angle of between 2 and 3 degrees.

For some applications, the optical system includes an objective lens 208that is configured to collect and collimate the rays reflected from thecurved surface of the object, such that the rays are directed towardelement 180. In addition, for some applications, element 208 acts asfirst narrow aperture, by only collecting rays within a narrow angle.Typically, the rays then pass through optical element 180, which acts asan additional narrow aperture.

For example, as shown in FIG. 3A, light is directed toward eye 210 froma broad angle of incidence. Optical element 180, which defines a narrowaperture, is disposed in any relevant conjugate plane, and is used toblock light from that reflects from the surface other than light withina narrow angle. In general, for such applications, optical element 180is an apodizer. For example, optical element may be an annular filter.Typically, at respective times, optical element 180 is used to allowreflected narrow-angle beams to pass through to the camera fromrespective angles of reflection, and to block reflected light from beingimaged by the camera outside of the reflected narrow-angle beam. Thus,at respective times, respective circular rings of the surface are imagedby the camera.

For some applications, illuminating the object (e.g., the eye) from abroad angle of incidence, but such that a respective narrow angleillumination beam is focused upon respective portions of the surface,and collecting the light using a light collection aperture having anarrow angle opening, enhances the ratio between the collected quantityof specular light and the collected quantity of non-specular light dueto surface topography. This is due to the fact that the quantity ofnon-specular light that is collected is proportional to the solid angleof the collecting optical system and its radiance does not depend on theangle of the reflecting surface relative to the angle of theillumination beam. By contrast, the quantity of specular light that iscollected depends on the solid angle of divergence of the beam itself(assuming that the solid angle of the collecting optical system islarger than the solid angle of divergence of the beam), and it alsodepends on the angle of the reflecting surface relative to the angle ofillumination beam and the optical axis of the collecting system.

Typically light that is reflected from the surface of the object isnarrowed by narrowing the angle of the reflected light using opticalelement 180, which is disposed at the imaging or collecting opticalsystem and/or any conjugate plane. The optical system is typically setto a narrow angle using the techniques described above, such that thespecular light is transmitted through optical element 180, while most ofthe non-specular light is blocked, and thus extra sensitivity tospecular light loss is achieved. When a gradual change in surfacetopography is present (due to sub-micron level variations in the heightof the surface), a portion of the specular light is reflected in adifferent direction from the rest of the specular light, such that theportion of the specular light is not collected by the aperture, and theamount of collected specular light is reduced. For some applications,the system is typically able to detect such sub-micron level variationsin the height of the surface (e.g., sub-micron level variations in theheight of the tear film surface) by detecting darkened regions in thereceived specular light.

Typically, a detectable change in the light that is received by thecamera is caused by a change in the surface angle of the object thatcorresponds to more than 10 percent of the angle of the light beam thatthat is received by the camera. Since the narrow-aperture opticalelement is configured to collect a light beam that defines an angle inthe order of 1-3 degrees, the system is sensitive to changes in theangle of the surface of less than 0.1-0.3 degrees. Typically, theresolution of the optical system is such that the camera pixel sizecorresponds to a distance of 5 microns across the surface of the object.Therefore, typically, the system is able to detect surface heightvariations of less than 25 nm (e.g., 10-25 nm), which correspond to asurface angle of 0.1-0.3 degrees over a 5 micron distance.

The above technique and system is schematically illustrated in FIGS. 3Aand 3B. As shown in FIG. 3A, the surface of the cornea of eye 210 isilluminated by a broad angle illuminating beam using focusing opticalelement(s) (e.g., lens 200) that are designed such that, out of theoverall angle of light rays that is directed to the cornea, rays withfrom respective angles are directed toward respective portions of thecornea. Due to the angle of the surface, at a given location, specularlight rays 204 that are reflected from the cornea are collected andcollimated by objective lens 208. The collected light passes throughnarrow aperture optical element, such that only the reflected light fromnarrow angle beam 202 passes through the optical element. The reflectedlight from narrow angle beam 202 is directed by the rest of the opticalsystem towards camera 270, such that the camera images a circular ringon the corneal surface upon which narrow angle beam 202 was incident. Inthis configuration, most of the non-specular light is blocked.

FIG. 3B illustrates the reflection structure of the specular light inthe presence of a pit or a bump on the portion of corneal surface of eye210 upon which narrow angle beam 202 was incident. (It is noted that,for illustrative purposes, lens 200 is not shown in FIG. 3B.) Theillumination light beam 202 illuminates the portion of the surface ofthe cornea. Most of the specular light rays that reflect from theportion of the surface are reflected toward objective lens 208 and thentoward the aperture in narrow aperture optical element 180. However,when a pit 212 is present, a portion 206 of light rays are reflected todifferent angles and they are not collected by objective lens 208,and/or narrow aperture optical element 180. Therefore, there is adarkened region corresponding to this location on the image. In responseto detecting darkened regions of the image of the surface, computerprocessor 28 (FIG. 1) determines the surface topography of the tear film(typically, to the sub-micron level).

In accordance with the description of FIGS. 3A-3B, the insertion ofoptical element 180 in the imaging path may increase the selectivity ofimaging to specular object surface reflection. In this manner, thesensitivity to lost specular light at a given radial position can beemphasized.

It is noted that, at the center of the surface of the object that isimaged (e.g., at the center of the cornea of eye 210), the system istypically unable to detect surface topography using the above-describedtechnique, since all of the reflected specular light is typicallyreceived by narrow-angle collecting optical element 208 and/or by narrowaperture optical element 180 (when the narrow aperture is in the centralportion of the optical imaging plane). Therefore, typically, opticalsystem 10 utilizes spectrometer 250 (FIG. 1) to measure the spectrum ofthe broadband light in the central region of the surface of the object(e.g., the cornea of the eye). The measured spectrum is analyzed, forexample, by comparing the spectrum to a model of electromagnetic wavesimulation. Typically, the computer processor is configured to detect anon-flat topography (e.g., due to collapse of the lipid layer asdescribed hereinabove) in the central region of the object, by analyzingthe measured spectrum. In this manner, the specular normal incidence ismeasured using the spectrometer, while the imaging system is configuredto have high camera sensitivity to the topography of the outer regionsof the surface, using the techniques described herein, due to theoblique incident light.

For some applications, an RGB camera (e.g., camera 270) is used todetect a non-flat topography (e.g., due to collapse of the lipid layeras described hereinabove) in the central region of the object.Typically, collapsed regions of the lipid layer of the tear film (whichmay form the appearance of a net on the tear layer, as describedhereinabove) changes the color of the lipid layer. This is because thehomogeneous area has a color that matches the average lipid thickness,while the collapsed regions contribute a different color ratio. Thiscontribution is proportional to the area of the collapsed regions. Forsome applications of the present invention, changes in the area of thecollapsed regions relative to the total area that is imaged isdetermined by measuring a change of the lipid layer's color as afunction of time using images obtained using an RGB camera. For someapplications, optical system 10 is configured to detect that that thelipid layer has a thickness of less than a given thickness (e.g., athickness of less than between 20 nm and 0 nm), by detecting that aregion of the RGB image is achromatic. Typically, using the techniquesdescribed herein, the computer processor of the optical system isconfigured to distinguish between darkening due to color based lipidthickness and darkening due to scattered light that is caused by surfacetopography.

In order to measure spectral measurements, such as the change of thecolor due to a layer thickness, a camera is used that has high spectralresolution qualities. As described hereinabove, when the collapsed lipidlayer forms a net structure, curves of the net of the lipid layertypically have widths of 10-30 microns. Each R, G, or B sub-pixel of thecamera typically has a pixel size that is less than 15 microns (e.g.,less than 10 microns), such that the resolution of each of the R, G, andB detectors is smaller than the thickness of the curves of the net.Further typically, the point-spread function of the RGB camera for eachof the colors is less than 15 microns (e.g., less than 10 microns). Forsome applications, the RGB camera's modulation transfer function ishigher than 0.9. Typically, by virtue of having a modulation transferfunction at such a value, the camera is configured to detect a colorvariation of about 1% between adjacent pixels, and/or is configured todetect at least a single gray level difference at 10 bit information.

Typically, the thicknesses of respective layers of the tear film varyover time due to a number of different mechanisms. For example, the tearfilm aqueous layer evaporates while the eyelids are open, thickens dueto tearing, and drains if overflow take place. For some applications, atear film measuring system (such as system 10, or a different systemthat may include any one of an interferometer, a spectrometer, and/or acamera) acquires measurements over a period of time, and parameters ofthe tear film are acquired by averaging the acquired measurements. Forexample, the thickness of the aqueous layer of the tear film as afunction of time, the flow rate of the aqueous layer, and/or theevaporation rate of the aqueous layer may be detected usingspectroscopic and/or interferometric measurements. As described infurther detail hereinbelow, for some applications, a plurality ofmeasurements are acquired over a time period of more than 1 secondand/or less than 10 seconds (e.g., less than 5 seconds), e.g., 1-10seconds, or 1-5 seconds. Typically, the measurement time of each of themeasurements is more than 20 milliseconds and/or less than 300milliseconds, e.g., 20-300 milliseconds.

In addition, since the rate of thickness change, and/or the evaporationrate of the aqueous layer changes with time, and there is also a liquidflow on top of the cornea, the interferometer and/or spectrometeracquires measurements at a minimal rate of measurements per second, asdescribed in further detail hereinbelow. Furthermore, since for healthypeople the rate of thickness change, and/or the evaporation rate istypically only several nanometers per second or lower, theinterferometer and/or spectrometer is configured to preserve therepeatability of the measurements on a sub-nanometer level.

As described above, the tear film aqueous layer evaporates while theeyelids are open, thickens due to tearing, and drains if overflow takeplace. Typically, the period of time over which measurements areacquired is long enough such as to limit the effect of randomfluctuations in the measured parameter, but short enough to excludebiological changes. For example, measurements of the thickness of alayer of the tear film may be acquired over a period of more than 1second and/or less than 10 seconds (e.g., less than 5 seconds), e.g.,1-10 seconds, or 1-5 seconds. During this period, several of theaforementioned mechanisms may occur. Typically, system 10 is configuredto differentiate between the various mechanisms and to thereby determinea parameter that relates to a particular mechanism.

Typically, a subject maintains their eye in a stable position, whilefocusing on a specific target, for a period of between severalmilliseconds and several seconds. Therefore, in order to perform each ofthe measurements such that the eye is in a stable position for theduration of the measurement, a measurement time of less than 300milliseconds is used for each measurement.

Furthermore, measurement of liquid dynamics (e.g., flow rate, rate ofthickness change, and/or evaporation rate) is typically performed in amanner that is such as to reduce smearing of the spectroscopic and/orinterferometric signal due to a change in the uniformity of the layersof the tear film. Typical flow rates 1-2 seconds after a blink are inthe order of 1 mm per second. Assuming that the non-uniformity permillimeter of the layer thickness is less than 200 nm, then in order toobtain a non-smeared signal using an average wavelength of 800 nm, atleast two measurements per second are typically acquired. For someapplications, in order to distinguish between different dynamicparameters, such as the flow rate and the evaporation rate, at a singlestatic location on the eye, at least 20 measurements per second areacquired.

The rate of thickness change, and/or the evaporation rate of the aqueouslayer is determined by the gradient of the thickness of the aqueouslayer as a function of time. Since the gradient itself varies due todifferent conditions of the lipids integrity, the level of humidity, thetemperature and air flow, the slope is a momentary characteristic of theaqueous layer. For some applications, the spectrometer and/orinterferometer is configured to acquire measurements at a frequency thatis such that the gradient can be determined. For example, for dry eyepatients that have a fast rate of thickness change, and/or evaporationrate and a blink rate in the order one blink every 5 seconds or faster,measurements are acquired at a frequency of at least 5 measurements persecond.

Evaporation itself during a given measurement can smear the measurement,if a significant thickness change occurs during the measurement.Typically, the maximal allowed change of the thickness of the aqueouslayer within one measurement should be less than one tenth of theaverage wavelength. For an average wavelength of 800 nm, the allowedchange in thickness during the measurement is 80 nm. Since dry eyepatients can have evaporation rates of 250 nm per second, typically theduration of each measurement is less than 300 msec. In order to providea sufficient signal-to-noise ratio, the duration of each measurement istypically greater than 20 milliseconds. Thus, typically, eachmeasurement is between 20 and 300 milliseconds in duration, and thereare more than 4 and/or less than 50 (e.g., 4-50) measurements persecond.

Reference is now made to FIG. 4, which is a graph showing a typicalvariation of the thickness of an aqueous layer as a function of time, asmeasured in accordance with some applications of the present invention.For some applications, the rate of thickness change and/or theevaporation rate of the aqueous layer is extracted from the curve shownin FIG. 4 by selecting a portion of the curve within which there is alinear correlation between the thickness of the aqueous layer and time,and determining the gradient of the curve within that portion. For someapplications, in order to determine the rate of thickness change, and/orthe evaporation rate, a portion of the curve that terminates at a giventime prior to a blink (e.g., 0.1 seconds prior to a blink) is selected.For some applications, measurements are performed within a period of 2seconds (e.g., between 0.2 seconds and 1 second) prior to a blink. Forexample, a time period starting 0.5 seconds before a blink andterminating 0.1 seconds before the blink may be selected. Alternatively,a portion of the curve is selected irrespective of the temporalproximity of the section of the curve to a blink, or a portion of thecurve is selected based on the portion starting a given time after ablink. For some applications, a portion of the curve is selected inresponse to (a) the gradient within the portion being negative (whichimplies layer thinning), and (b) the correlation value (R^2) of thecurve being greater than a threshold, such a greater than 0.8, orgreater than 0.9. For some applications, only a portion of the curvethat corresponds to four or more measurements and that satisfies theaforementioned criteria is selected for determining the rate ofthickness change, and/or the evaporation rate. (It is noted that forsome applications, algorithmic operations that are the equivalent ofperforming the above steps are performed, without actually plotting acurve as shown in FIG. 4.)

For example, as shown in FIG. 4, for the portion of the curve betweenapproximately 1.65 seconds and 2.05 seconds, the curve corresponds tofour or more measurements, has a negative gradient, and has acorrelation rate of more than 0.9. This portion of the curve terminatesshortly before a blink at around 2.15 seconds. Therefore, the rate ofthickness change, and/or the evaporation rate may be determined bydetermining the gradient of the curve within that portion of the curve.As shown, in the graph of FIG. 4, seven aqueous layer thickness datapoints were selected, as indicated by the data points with a circlearound them. By contrast, other data points do not satisfy theaforementioned criteria, for example, due to disruptive events (e.g.,fast fluid flow, tearing, partial blinks, or eye movements).

Reference is now made to FIG. 5, which is a graph showing the thicknessof the lipid layer of the tear film as function of time, as measured inaccordance with some applications of the present invention.

For some applications of the present invention, the stability of abiological layer, such as a layer of the tear film (e.g., the lipidlayer) is used to determine the health of the layer, and/or the healthof the tear film. For some applications, repeated measurements areperformed on such a layer in order to determine the stability level ofthe layer, and the health of the layer, and/or the health of the tearfilm is determined in response thereto. For example, spectrometricand/or interferometric measurements as described herein may beperformed. For some applications, optical system 10 (shown in FIG. 1) isused to perform such measurements.

For some applications, the stability level of the layer is determined bydetermining a ratio between the variance or standard deviation of aparameter of the layer (such as, the layer thickness) and the averagevalues of the parameter. Since, for example, the variance or standarddeviation of the layer thickness is affected by the actual value of thelayer thickness, determining the aforementioned ratio is a way ofstandardizing the variance or standard deviation measurements. Forexample, for the measurements of the lipid layer thickness, shown inFIG. 5, the standard deviation of the measurements is 1.4 nm, and theaverage thickness is 31.6 nm. Therefore, in accordance with thetechniques described herein, the layer stability is defined as 4.4percent (i.e., the standard deviation of the thickness as a percentageof the average thickness).

For some applications, while measuring the stability, disruptive eventssuch as blinking are identified, and measurements are classified inrelation to these events. For example, the flow rate of a substancewithin a layer is likely to change dramatically following such adisruptive event and to settle thereafter. Therefore, for someapplications, in order to determine the intrinsic stability of aparameter of a layer (i.e., the stability of the layer in the absence ofthe effects of disruptive events), measurements of the parameter thatare performed soon after such an event are discarded. For example, fordetermining the stability of a parameter (e.g., thickness) of a layer ofthe tear film (e.g., the lipid layer) measurements that are acquiredimmediately after a blink are not used, and measurements that areacquired shortly before a subsequent blink are used. It has been foundby the inventors of the present application that for healthy patients,the normal intrinsic stability parameter variations of the thickness ofthe lipid layer of the tear film are typically smaller than 10% (e.g.smaller than 5%), whereas for non-healthy patients intrinsic stabilityparameter variations are substantially higher.

Breakup of a layer's integrity can happen from time to time and naturalmechanisms, such as blinking, may repair the layer subsequent to thebreakup. Many mechanisms can lead to break up the layer's integrity,especially in a complex multi layers fluid, like the tear film.Measuring the time from a resetting event, such as a blink, to thebreakup event provides a characteristics parameter of the layerintegrity.

For some applications of the present invention, a breakup event isidentified, in response to identifying that the layer thickness at aspecific area crosses a predefined threshold. In response thereto, abreakup time is determined by determining the time that elapsed sincethe previous resetting event (e.g., the previous blink) until thebreakup. For some applications, the level of health of the layer of thetear film, and/or a level of health of the tear film is identified inresponse to the determined breakup time. Typically, only the firstbreakup after the resetting event is considered for breakup timecalculation. For some applications, the aforementioned threshold isdefined relative to a moving average of the layer's thickness over aprevious given time period. For example, the moving average may bedetermined based upon measurements performed over the previous 1 second,or over the previous 0.5 seconds. Typically, the moving average isdetermined based upon measurements performed over a period of time thatis between 0.5 and 1 second. For some applications, the threshold isdefined as a percentage of the moving average. For example, thethreshold may be 90 percent, or 95 percent of the moving average, suchthat if the thickness falls by 10 percent or 5 percent relative to themoving average, a breakup event is identified as having occurred.

For example, the curve shown in FIG. 5 is a moving threshold, which isbased upon a moving average of the lipid layer thickness, based upon theprevious 0.6 seconds of measured data. For the threshold shown in FIG.5, the threshold is 90 percent of the moving average. At approximately18.7 seconds, there is a blink, which is a disruptive event.Subsequently, at approximately 19.4 seconds, 0.685 seconds after theblink, the measured thickness drops below the threshold curve, which isindicative of a breakup event having occurred. Hence, the breakup timeis 0.685 seconds.

For some applications, in identifying breakup events, the shifting offluid within a layer is accounted for. Since the fluid is constantlyshifting, there is a thickness change over time. It is noted that thecharacteristic span of thickness values across a wide enough area istypically similar at different points in time. However, the thickness ina specific area can change dramatically if a breakup event occurredwithin the area, or if a collapsed area spreads into the measurementarea from the surrounding areas. Therefore, for example, the thicknessesof surrounding areas may be taken into account, in order to determinewhether a breakup event has occurred in a given area. For example, thevariance in the thicknesses of areas in the vicinity of the given areamay be compared to the variance in the given area, and a breakup eventmay be confirmed as having occurred at least partially in responsethereto.

For some applications, breakup events are detected in response todetecting the histogram of the ratios between R, G, and B of the colorcamera at respective areas. Since pixel colors have a relation tothickness, the histogram of the ratios between R, G, and B of the cameraat an area that experienced a breakup event undergoes a substantialchange, whereas at areas that did not experience a breakup event thehistogram remains substantially the same.

For some applications, the layer stability is determined at leastpartially in response to determining the number of breakup events thatoccur between resetting events.

Typically, the layer stability is determined by performing depthmeasurements on fewer than 10, e.g., fewer than 5 areas within thelayer. Typically, each such depth measurement is averaged over an areaextending over more than 0.1 square millimeters and/or less than 0.2square millimeters, e.g., 0.1-0.2 square millimeters. Typically,thickness measurements are performed at time intervals of more than 20msec, and/or less than 300 msec (e.g., every 20-300 msec) in order todetermine such breakup events.

Applications of the invention described herein can take the form of acomputer program product accessible from a computer-usable orcomputer-readable medium (e.g., a non-transitory computer-readablemedium) providing program code for use by or in connection with acomputer or any instruction execution system, such as a computerprocessor 28 (FIG. 1), which may be in communication with optical system10. For the purpose of this description, a computer-usable or computerreadable medium can be any apparatus that can comprise, store,communicate, propagate, or transport the program for use by or inconnection with the instruction execution system, apparatus, or device.The medium can be an electronic, magnetic, optical, electromagnetic,infrared, or semiconductor system (or apparatus or device) or apropagation medium. Typically, the computer-usable or computer readablemedium is a non-transitory computer-usable or computer readable medium.

Examples of a computer-readable medium include a semiconductor or solidstate memory, magnetic tape, a removable computer diskette, a randomaccess memory (RAM), a read-only memory (ROM), a rigid magnetic disk andan optical disk. Current examples of optical disks include compactdisk-read only memory (CD-ROM), compact disk-read/write (CD-R/W) andDVD.

A data processing system suitable for storing and/or executing programcode will include at least one processor (e.g., computer processor 28)coupled directly or indirectly to memory elements through a system bus.The memory elements can include local memory employed during actualexecution of the program code, bulk storage, and cache memories whichprovide temporary storage of at least some program code in order toreduce the number of times code must be retrieved from bulk storageduring execution. The system can read the inventive instructions on theprogram storage devices and follow these instructions to execute themethodology of the embodiments of the invention.

Network adapters may be coupled to the processor to enable the processorto become coupled to other processors or remote printers or storagedevices through intervening private or public networks. Modems, cablemodem and Ethernet cards are just a few of the currently available typesof network adapters.

Computer program code for carrying out operations of the presentinvention may be written in any combination of one or more programminglanguages, including an object oriented programming language such asJava, Smalltalk, C++ or the like and conventional procedural programminglanguages, such as the C programming language or similar programminglanguages.

It will be understood that the algorithms described herein, can beimplemented by computer program instructions. These computer programinstructions may be provided to a processor of a general purposecomputer, special purpose computer, or other programmable dataprocessing apparatus to produce a machine, such that the instructions,which execute via the processor of the computer (e.g., computerprocessor 28) or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the algorithmsdescribed in the present application. These computer programinstructions may also be stored in a computer-readable medium (e.g., anon-transitory computer-readable medium) that can direct a computer orother programmable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablemedium produce an article of manufacture including instruction meanswhich implement the function/act specified in the flowchart blocks andalgorithms. The computer program instructions may also be loaded onto acomputer or other programmable data processing apparatus to cause aseries of operational steps to be performed on the computer or otherprogrammable apparatus to produce a computer implemented process suchthat the instructions which execute on the computer or otherprogrammable apparatus provide processes for implementing thefunctions/acts specified in the algorithms described in the presentapplication.

Computer processor 28 is typically a hardware device programmed withcomputer program instructions to produce a special purpose computer. Forexample, when programmed to perform the algorithms described herein,computer processor 28 typically acts as a special purposetear-film-analysis computer processor. Typically, the operationsdescribed herein that are performed by computer processor 28 transformthe physical state of a memory, which is a real physical article, tohave a different magnetic polarity, electrical charge, or the likedepending on the technology of the memory that is used. For someapplications, operations that are described as being performed by acomputer processor are performed by a plurality of computer processorsin combination with each other.

It should be noted that features that are described with reference toone or more embodiments are described by way of example rather than byway of limitation to those embodiments. Thus, unless stated otherwise orunless particular combinations are clearly inadmissible, optionalfeatures that are described with reference to only some embodiments areassumed to be likewise applicable to all other embodiments also.

The invention claimed is:
 1. A system for performing structure measurement on a tear film of an eye of a subject, the system comprising: a broadband light source configured to illuminate at least a portion of a surface of the tear film; a spectrometer configured to measure a spectrum of light of the broadband light that is reflected from at least one point of the tear film; a color camera configured to obtain color information for a plurality of points of the tear film, by imaging a field of view of the tear film; and a processing unit coupled to the camera and to the spectrometer, the processing unit being configured to: receive data from the color camera that are indicative of a characteristic of the tear film, receive data from the spectrometer that are indicative of a characteristic of the tear film, and based upon a combination of the data received from the color camera and the data received from the spectrometer, generate an output indicative of a structure of the tear film.
 2. The system according to claim 1, wherein the system is configured such that image planes of the spectrometer, the camera and the surface of the tear film are conjugate to each other.
 3. The system according to claim 1, further comprising an illumination system that is configured to trace rays of light from the broadband light source such that a respective narrow angle illumination beam is focused upon respective portions of the surface of the tear film.
 4. The system according to claim 1, wherein the portion of the surface is non-planar, the system further comprising an illumination system that is configured to direct the broadband light toward the non-planar portion.
 5. The system according to claim 1, wherein the portion of the surface is curved, the system further comprising an illumination system that is configured to direct the broadband light toward the curved portion.
 6. The system according to claim 1, further comprising a mechanism for centering a cornea of the subject's eye onto optical axes of the spectrometer and the color camera, the mechanism comprising a projector that is configured to project a given pattern onto the subject's cornea.
 7. The system according to claim 1, wherein the spectrometer is configured to measure the spectrum of reflected light from the at least one point of the tear film using a measurement time of between 20 and 300 milliseconds.
 8. The system according to claim 1, further comprising an aperture that is configured to facilitate detection of tear film sub-micron level surface topography by causing narrow angles of light of the broadband light that is reflected from the tear film to be received by the color camera.
 9. The system according to claim 1, wherein the processing unit is further configured to compute respective rates of change of thicknesses of one or more layers of the tear film at different points along the tear film obtained over a known time interval.
 10. The system according to claim 1, further comprising one or more optical elements selected from the group consisting of: lenses, Fresnel lenses, annular aperture filters, polarizers, spatial light modulators (SLMs), and diffractive optical elements (DOEs).
 11. The system according to claim 1, wherein the spectrometer is configured to measure the spectrum of reflected light from the at least one point of the tear film using a measurement spot size of between 100 microns and 240 microns.
 12. The system according to claim 1, further comprising an autofocusing mechanism that is configured to focus the color camera and the spectrometer.
 13. The system according to claim 12, wherein the autofocusing mechanism comprises elements configured to be imaged onto a cornea of the subject's eye, such as to facilitate positioning the cornea by examining a sharpness of the images of the elements on the cornea.
 14. The system according to claim 1, wherein the spectrometer further includes a deflecting element for scanning different tear film locations.
 15. The system according to claim 14, further comprising a narrow band filter between the light source and the camera for producing interference fringe patterns characteristic of different layers of the tear film.
 16. The system according to claim 15, wherein the processing unit is configured to use the spectrometer to correlate a fringe pattern in a known location or pixel with a respective thickness of each layer of the tear film and thereby derive the respective thickness of each layer of the tear film without scanning the tear film.
 17. A method for performing structure measurement on a tear film of an eye of a subject, the method comprising: illuminating at least a portion of a surface of the tear film using a broadband light source; using a spectrometer, measuring a spectrum of light of the broadband light that is reflected from at least one point of the tear film; obtaining color information for a plurality of points of the tear film, by imaging a field of view of the tear film using a color camera; and using a processing unit: receiving data from the color camera that are indicative of a characteristic of the tear film; receiving data from the spectrometer that are indicative of a characteristic of the tear film; and based upon a combination of the data received from the color camera and the data received from the spectrometer, generating an output indicative of a structure of the tear film.
 18. The method according to claim 17, wherein: measuring the spectrum of light of the broadband light that is reflected from at least one point of the tear film using the spectrometer comprises measuring the spectrum of light of the broadband light that is reflected from at least one point of the tear film using a spectrometer having an image plane that is conjugate with an image plane of the surface of the tear film; and imaging the field of view of the tear film using the color camera comprises imaging the field of view of the tear film using a color camera having an image plane that is conjugate with the image plane of the surface of the tear film and with the image plane of the spectrometer.
 19. The method according to claim 17, further comprising computing respective rates of change of thicknesses of one or more layers of the tear film at different points along the tear film obtained over a known time interval.
 20. The method according to claim 17, further comprising: (i) using a narrow band filter to obtain an interference pattern due to thickness non-uniformity of one or more layers of the tear film; (ii) determining absolute values of thicknesses of the one or more layers of the tear film at overlapping locations of the interference pattern and the at least one spectrometer measurement point; and (iii) applying the absolute values at the overlapping locations to determine the absolute values of the thicknesses at neighboring pixels that also have interference patterns.
 21. The method according to claim 17, further comprising using electromagnetic simulation in order to fit measurement results to a biological model of stack of tissues and liquid layers.
 22. The method according to claim 17, further comprising autofocusing the color camera and the spectrometer. 